The present disclosure relates to a photosensitizer and a photoelectric conversion device using the photosensitizer.
Organic solar cells, which have been attracting attention in recent years as next-generation solar cells, are roughly classified into an organic thin-film type and a dye-sensitization type. The organic thin-film solar cell utilizes a p-n junction created by combining organic materials, and its work mechanism is exactly the same as that of an inorganic solar cell, typified by silicon. In contrast, the dye-sensitized solar cell (Dye Sensitized Solar Cell: DSC) is characterized by use of a modified electrode in which an organic dye, as a photosensitizer, is combined with an inorganic semiconductor, such as titanium oxide and zinc oxide. Further, the work mechanism of the dye-sensitized solar cell is also completely different from that of an inorganic solar cell, and rather close to photosynthesis. Such a solar cell, which may be regarded as an inorganic-organic hybrid molecular device, is drawing attention, particularly, as a low-cost solar cell (Brian O'Regan and Michael Gratzel, “A low-cost, high-efficiency solar cell based on dye-sensitized colloidal TiO2 films”, Nature, Vol. 353, pp. 737-740, 1991 (Non-Patent Document 1)).
Further, all the conventional solar cells have a monochromatic hue and fail to have excellent design characteristics. In contrast, the dye-sensitized solar cells are producible in various hues, such as red, blue and yellow, just by changing the kind of dye. Such a wide variation in color gives also very excellent design characteristics, and use for various purposes and their market growth are expected.
The working electrode of the dye-sensitized solar cell is produced by depositing an inorganic semiconductor, such as titanium oxide and zinc oxide, on a conductive substrate by sintering, and by causing the inorganic semiconductor to adsorb sensitizing dye. Since the dye-sensitized solar cell has simple structure in which space between the working electrode and a counter electrode, such as platinum, is filled with iodine-based electrolyte solution, manufacture is easy. Further, since no special equipment, such as a vacuum line, is required in manufacture, reduction in cost is easier than conventional solar cells. Especially, a so-called Gratzel DSC, which requires no investment on special equipment as well as using low cost materials for an electrode and the like, has a high probability of greatly reducing the cost. As the electrode of the Gratzel DSC, porous titanium oxide having a high roughness factor, and which has been produced by sintering nanoparticles at high temperature, is used. Currently, high conversion efficiency of 12% or higher is achieved by combining a ruthenium dye, as photosensitizer, with the porous titanium oxide. Therefore, production for sale in the near future is expected.
However, there remain problems to be solved also for practical use of such a Gratzel DSC. For example, most of dyes currently having high conversion efficiency of practical use level are ruthenium complex (for example, Japanese Patent No. 3731752 (Patent Document 1)). Since ruthenium is a rare metal, the reserve of which is small, there is a problem as to the resource. Further, there is also a problem of characteristic toxicity of metals as well as a significant problem in cost. To solve these problems, metal-free organic dye is intensively developed worldwide, but the current situation does not reach practical use level as to the conversion efficiency and the durability (for example, Japanese Patent No. 4080288 (Patent Document 2)). Further, most of these high performance organic dyes are yellow through reddish purple. Therefore, development of blue dye with high performance and high durability, which absorbs a longer wavelength region, is strongly requested.